Acanthosis nigricans is a commonly occurring skin lesion which indicates endogenous hyperinsulinemia. We hypothesize that patients with this skin lesion have a variety of hereditary defects in insulin action, all of which are associated with a similar phenotype and a high risk for noninsulin dependent diabetes mellitus. Subjects will have insulin responsiveness evaluated annually, and selected severely affected subjects and their parents will have evaluation of the insulin receptor gene for point mutations.